For marking height top part of building (building=*) or part of building (building:part=yes) above ground level we use tag height=*. If building or part of building starts not from the ground level, we should use tag min_height=* for describing height of bottom part of building or part of building. By default meaning is counted in meters.

If you know number of levels of building or part of building above the ground level, it is also useful to add tag building:levels=* (and building:min_level=*, if the bottom level of building or part of building is not located on the ground).